In the combustion of fossil fuels, which is carried out with the aim of energy recovery, and in the incineration of refuse, sewage sludge and industrial waste, which is carried out with the aim of waste disposal and energy recovery, flue gases are formed that are contaminated by heavy metals, heavy-metal compounds, and other pollutants (e.g., ash, SO2, SO3, and the like). The contaminants are present in the flue gas in different amounts depending on the particular combustion process, and the concentration of the contaminants undergoing certain variations during each combustion process. For example, the different and varying composition of refuse, industrial waste and sewage sludge means that the flue gases formed on combustion of this waste are contaminated by different amounts of environmentally polluting substances present in the starting material. However, all contaminants must be substantially removed from the flue gases before the latter can be released into the atmosphere, since very many contaminants exert toxic effects on humans, animals and plants even in low concentration.
Flue gases are treated with a variety of devices to remove pollutants before the gas is introduced into the air. Solids (e.g., fly ash) are typically removed by electrostatic precipitators and in so-called baghouses, while liquid contact scrubbers remove gaseous and volatile wastes from the flue gas before it is discharged. For example, while flue gas desulfurization units are specifically designed to remove gaseous sulfur compounds (e.g., SO2 and SO3) from flue gas, the contact liquid traps many pollutants like heavy metals. Trapped solids and contaminated liquid wastes are typically disposed of in pits and/or waste lagoons. The approximately 1.05 billion tons of coal burned each year in the United States contain significant amounts of toxic wastes. On top of emitting 1.9 billion tons of carbon dioxide each year, coal-fired power plants in the United States also create 120 million tons of waste each year. That means each of the nation's ˜500 coal-fired power plants produces an average 240,000 tons of waste each year. A power plant that operates for 40 years will leave behind ˜9.6 million tons of waste.
Many of the heavy metals that are released as products of combustion from power plant, incinerators, and the like are environmentally and biologically toxic elements, such as lead, mercury, nickel, tin, cadmium, antimony, and arsenic. Waste that is stored in ponds or pits can present health and environmental risks if released into ground water. Mercury, for instance, is a persistent environmental pollutant that is trapped by organisms at the low end of the food chain and concentrated by organisms, such as fish and birds, higher up the food chain. When fish and animals eat these microorganisms, the toxins accumulate and can interfere with reproduction, growth, and behavior, and can even cause death. According to the Centers for Disease Control and Prevention, in 2003 eight percent of American women of childbearing age had unsafe levels of mercury in their blood, putting approximately 322,000 newborns at risk of neurological deficits. Mercury exposure also can lead to increase cardiovascular risk in adults.
There exists a need in the art for more effective means for remediation of heavy contaminants present in combustion wastes.